There has been a need for mounting an illuminance sensor device for a liquid crystal panel of devices such as digital cameras and mobile phones, in order to control the amount of light emitted from a backlight device of the liquid crystal according to the illuminance of disturbance light. Analog type illuminance sensor devices were used before, but nowadays digital-type illuminance sensors are common because high resolution is required. Further, the illumination sensor devices are required to have spectral characteristics close to visual sensitivity. Thus, for the illuminance sensor devices having an analog to digital converter that converts an input current from photodiodes, there is a demand for realizing spectral characteristics close to the visual sensitivity with simple configuration.
In conventional illuminance sensor devices, in order to realize the spectral characteristics close to the visual sensitivity, a scheme of subtracting between currents of a plurality of photodiodes having different spectral characteristics is generally adopted.
As such a scheme of subtracting between currents of a plurality of photodiodes, JP 2007-73591 (PTL 1) teaches that subtraction between the currents from the photodiodes having different spectral characteristics is performed using a current mirror circuit to thereby obtain spectral characteristics close to the visual sensitivity.
Also, as a scheme of subtracting between the currents from the photodiodes and obtaining different spectral characteristics through color filters, JP 2010-153484 (PTL 2) discloses that subtraction between currents from photodiodes having the different spectral characteristics is performed by a current mirror circuit and in addition color filters are used, whereby spectral characteristics close to the visual sensitivity are obtained.
In a sensing method generally adopted in the illuminance sensor devices, a sensor output is converted to a digital value by an analog to digital converter. Conversion of the sensor output to a digital value facilitates the processing by software in CPUs and microcomputers. Integral type analog to digital converters are capable of realizing a highly accurate resolution with a simple structure. The integral type analog to digital converters are suitable for devices which are required to have a slow but high resolution (16 bits or so), such as illuminance sensors.
As a first conventional example, FIG. 13 shows a subtractive type structure using a current mirror circuit as in JP 2007-73591 (PTL 1) and JP 2010-153484 (PTL 2). In FIG. 13, PD1 indicates a photodiode having spectral characteristics of the infrared region, PD2 indicates a photodiode having spectral characteristics of from the visible to the infrared region, and Q1 and Q2 indicate transistors constituting a current mirror circuit.
In FIG. 13 showing the first conventional example, input current from the photodiode PD1 having the spectral characteristics of the infrared region is represented as Iin1, and input current from the photodiode PD2 having the spectral characteristics of from the visible to the infrared region is represented as Iin2. In the first conventional example, a current corresponding to an amount of the input current Iin1 is subtracted from the input current Iin2 to calculate an amount of current (Iin2−Iin1×a) to thereby obtain spectral characteristics close to visual sensitivity characteristics.
FIG. 14 shows, as a second conventional example a configuration in which a sensor output is converted to a digital value by analog to digital converters ADC1, ADC2 and then subtraction between the digital values is performed. In FIG. 14 showing the second conventional example, input current from the photodiode PD1 having the spectral characteristics of the infrared region is represented as Iin1, and input current from the photodiode PD2 having the spectral characteristics of from the visible to the infrared region is represented as Iin2.
In the second conventional example, with a result of analog to digital conversion of the input current Iin2 by the analog to digital converter ADC1 being a digital value ADCCUNT2 and with a result of analog to digital conversion of the input current Iin1 by the analog to digital converter ADC2 being a digital value ADCOUNT1, the digital value ADCOUNT1 multiplied by a is subtracted from the digital value ADCOUNT2, so that the same result as in the first conventional example is obtained through the digital operations, as shown below.ADCOUNT2−ADCOUNT1×a=Iin2−Iin1×a 
FIG. 15 shows, as a third conventional example, a configuration in which analog to digital conversion is performed using one analog to digital converter ADC1 upon each of the input current Iin1 from the photodiode PD1 having the spectral characteristics of the infrared region and the input current Iin2 from the photodiode PD2 having the spectral characteristics of from the visible to the infrared region, followed by the subtraction between digital values obtained by the conversion. In the configuration of FIG. 15, unlike the second conventional example, the input currents Iin1 and Iin2 are not measured or determined at the same time, and their analog to digital conversions are performed at different times, i.e., in different conversion periods. Switching the input of the analog to digital converter ADC1 between the input current Iin1 and the input current Iin2 every conversion period allows the one analog to digital converter ADC1 to obtain the AD operation results.